Direct electrostatic printing requires special paper having a conductive base covered with a thin dielectric film. An electrostatic latent image is created on the special paper by selective ion discharge in air by means of a row of small electrodes which are raised to a high tension. The latent image is linked (for example by means of a magnetic brush conveying ink powder) and then fixed on the special paper by the application of pressure and/or heat.
In order for ion discharge to take place at the tips of the electrodes and in order for the ions to be deposited on the special paper, high voltage is set up between at least one of the electrodes and at least one counterelectrode placed in the proximity thereof so that the resulting electric field at the electrode extends perpendicularly to the surface of the special paper. In a prior embodiment, the counterelectrodes are placed on the same side of the special paper as the electrodes, and the discharge takes place by virtue of the capacitive effect of the counterelectrodes through the dielectric film of the special paper and by virtue of the conductive effect of the conductive base of said special paper.
The individual electrodes used in the print head are very small and very numerous for an image line of given length, for example, electrodes may be provided to print at a pitch of eight points per millimeter.
The high voltage required for ion discharge is preferably shared between the electrodes and the counterelectrodes, and since discharge takes place only above a threshold high value, the voltage applied solely to the electrodes or solely to the counterelectrodes is taken to be too small to be capable of setting up a discharge, and thus of printing, on its own.
The electrodes are aligned along at least one row and are organized in packets, with the electrodes in the same packet and the same row being interconnected. The counterelectrodes are distributed alongside the packets.
A relatively large gap, for example 0.2 mm to 0.5 mm, is required between adjacent counterelectrodes by virtue of the values of the voltages which are to be switched. This has an unfavorable effect on the printing when high resolution is required. In spite of the conductivity of the conductive base in the special paper, the electric field falls off in zones where the electrodes are disposed opposite a gap between counterelectrodes.
This drawback can be minimized by positioning the electrodes in two networks, each comprising the same number of packets in alternation, and by using counterelectrodes whose positions are offset along the packets by half a packet's length so that they overlap two consecutive half packets, with the counterelectrodes overlapping each half of a packet being switched simultaneously.
When writing an indirect electrostatic image, the latent image is written onto a thin insulating film and is then developed by being inked in exactly the same way as described above. The developed image is then transferred to a sheet of ordinary paper and is fixed thereon, for example by pressing the sheet of paper against the inked insulating film.
Although the disposition of the electrodes which is suitable for direct electrostatic writing is also suitable for indirect electrostatic writing, the same is not generally true for the disposition of the counterelectrodes since in the indirect case neither the insulating film nor the paper are conductive.
Consequently, the conventional solution in indirect electrostatic writing consists in causing the insulating film on which the latent image is deposited to pass between facing electrodes and counterelectrodes.
However, this solution has drawbacks, related in particular to he complexity of the system for applying voltage control signals to the overall assembly, and to the need for highly accurate positioning of the counterelectrodes relative to the electrodes in spite of the fact that it is advantageous with this indirect printing solution to able to separate the electrodes and the counterelectrodes in order to insert the insulating film or to replace it each time the user needs to change film.
It is thus preferable from this point of view for the electrodes and the counterelectrodes to be located on the same side of film.
The present invention therefore proposes a print head for printing an electrostatic image on a recording medium, in particular of the ordinary paper type, in which the electrodes and the counterelectrodes required for ion discharge are disposed on the same side of the recording medium.